Control characteristics of a driving force generated by a prime mover in response to an operation of an accelerator, a speed ratio established by a transmission, a shift timing, a steering angle of front wheels with respect to a turning angle of a steering wheel, a damping characteristic of a suspension and so on are determined depending on a type of the vehicle. However, a driving preference of the driver (i.e., driver's disposition or intention) depending on a running condition such as a road condition varies from person to person and hence designed control characteristics may not meet driver's preference in every situation.
In the conventional art, control characteristics of acceleration and steering and so on can be changed by selecting a driving mode from sporty mode, a normal mode, and a mild mode, and those characteristics may be adjusted in line with the driver's preference. For example, PCT international publication WO 2013/011572 describes a vehicle control system configured to detect or judge desired sportiness of the driver based on an acceleration and a steering angle. Based on judgement result, the control system increases an engine power with respect to an operating amount of an accelerator, changes a speed ratio in such a manner to expand a high speed region where an engine speed is high, and increases a hardness of a suspension.
The vehicle control system taught by WO 2013/011572 is configured to adjust the control characteristics of the vehicle based on an index representing the driver's preference determined based on a longitudinal acceleration, a synthesized acceleration of longitudinal acceleration and lateral acceleration, an absolute value of acceleration, or an estimated value of acceleration. According to the teachings of WO 2013/011572, specifically, an instant SPI is determined based on an instant value of acceleration detected continuously, and the instant SPI is updated when the current SPI exceeds the previous SPI to be used to calculate a command SPI. By contrast, if the instant SPI is fluctuating below the command SPI for a certain period of time, the command SPI is maintained until a predetermined condition is satisfied, specifically, until an integral deviation therebetween exceeds a threshold value, and then the instant SPI is lowered. That is, the command SPI calculated based on the instant SPI is increased immediately with an increase of the instant SPI, but lowered after a delay with respect to a drop of the instant SPI. Thus, the vehicle control system taught by WO 2013/011572 is configured to adjust the control characteristics of a relation between an accelerator opening and an engine torque, an assist amount of steering, or a damping characteristic of the suspension based on the command SPI.
However, acceleration of the vehicle is easily to be generated and sensed at a low speed, but difficult to be generated and sensed at a high speed. Therefore, if the index is calculated based on the (longitudinal) acceleration irrespective of vehicle speed, a determination of the fact that the driver prefers a sporty driving would be satisfied too easily at a low speed, and would be satisfied too difficult at a high speed. Consequently, a determination accuracy of the driving preference would be deteriorated. In order to avoid such disadvantage, according to the teachings of WO 2013/011572, the instant value of longitudinal (or forward) acceleration to be used to calculate the index is weighted to be increased at a low speed as compared that at a high speed.
According to the teachings of WO 2013/011572, accuracy of adjusting the control characteristics based on the driver's preference can be improved by thus correcting the acceleration in accordance with vehicle speed. However, an accelerator pedal is depressed and returned even during cruising at a constant speed to adjust a driving torque against a road load and air resistance. Specifically, the accelerator pedal is depressed when the driver feels that the vehicle speed is lowered, and then returned when the driver senses increase in acceleration. Such operations of the accelerator to adjust acceleration may be reflected on the index representing sportiness of driving. For example, in the above-explained vehicle control system using the command SPI, the command SPI is prevented from being lowered easily if a deviation between the command SPI and the instant SPI fluctuating below the command SPI is small. This is because the acceleration is increased to a certain extent and hence the control system determines a fact that the driver demands a sporty running.
As shown in FIG. 11, a minimum value of a change in acceleration that can be sensed by the driver during cruising at a constant speed is larger in a high speed range as compared to that in a low speed range. Therefore, the accelerator would be unintentionally opened widely by the driver during cruising at a high speed. That is, such operation of the accelerator to keep the vehicle cruising would be erroneously reflected on the index representing the driving preference in the high speed range. Consequently, the index would not be lowered even under the situation to be lowered and hence the control characteristics would be deviated from the driver's intention.